Pop quiz: what was the correct size of an OK button back in Mac OS 8? If you answered “I have no idea,” you would be in the same boat I was only a few minutes ago. It turns out that the correct size for your OK and Cancel buttons on Mac OS 8 was 58px by 20px. Unsurprisingly, the only way to figure out the answer to this question is to dig into Apple’s HIG, assuming you think about this (admittedly) nitpicky issue.

OK, next question: what’s the correct size of an OK button on OS X? You might think to open up everyone’s favorite UI design tool, Interface Builder. Alright, let’s start by creating a new Cocoa window and dragging a button onto it. Change its title to “OK” and take a look at the size: ah, 70px by 20px. Case closed, right? Well, no actually, it isn’t.

Next step: Create a new Carbon window and drag a button onto it. Rename the button to “OK” and take a look at its size. Uh-oh, this button is 69px by 20px. Well, so what? What’s one pixel between friends, after all?

So here’s the issue: Interface Builder for both Carbon and Cocoa disregards the Apple Human Interface Guidelines in this regard. Apple states that:

The standard width for OK and Cancel buttons is 68 pixels

Oops. In contrast, this is one place where UI design tools for Windows get it right. Go take a look at the Windows Ux Guidelines. They specify that buttons should be 50x14 Dialog Units (DLUs) or 75x23px. Drag a button onto a Winform in Visual Studio. It’s 75x23px. Drag a button onto a VC-generated RC file dialog: it’s 50x14DLU.